WO 98/10855 A1 discloses porous structures and processes for the manufacture thereof. In one embodiment, an apparatus comprises a movable belt, providing a flat surface, the belt being driven by a motor. Above the surface is a first hopper which dispenses a first type of media, which is uniformly laid out by a doctor blade or roller. A second hopper is provided downstream from the first, to dispense an intermediate layer of media, which is again uniformly laid out by a roller. Further downstream is a third hopper, which dispenses a third type of media, which is also uniformly laid out by a roller. As a result, a product is formed which can be densified on the belt by means not shown to form the green product, or subsequently densified. It is disclosed that one could also form multi-layered products with two or more zones or layers of different character by sequentially compressing an added layer over a previously made flat structure. To do so would merely require the step of feeding a subsequent layer of another media composite, prior to heating, followed by densification of all the layers.
WO 95/03019 A1 discloses a manufacturing apparatus comprising a production line fed by three reels of a web-like containing and supporting material. An amount of absorbent material is deposited onto the web material from the reels at a depositing station. The deposition is achieved by means of a dispenser comprising two containers arranged in sequence from which the materials are poured. The absorbent material is blended in advance with a bonding agent, in particle, fibre or powder form, which can be activated by application of heat. Immediately downstream of the deposition station, rollers are provided for locally removing super-absorbent polymers (SAP) or other absorbent material. Then the web passes through a sticking station provided with means for applying heat such as, for example, an infrared oven, or heaters capable of blowing hot air, or other known devices. At a subsequent gluing station, a dispenser applies longitudinal lines or stripes of an adhesive material onto the web. Downstream of the dispenser, two calibrating or pressure rollers join together the web-like sheet from a further reel and the sheet, thus covering and enclosing between them the absorbent materials without any appreciable compression thereof. The joining of the two sheets occurs along the adhesive stripes. Downstream of the rollers, the apparatus provides for an arrangement that is substantially similar to the one already illustrated, with a deposition station, rollers for locally removing the SAP material, a sticking station and a gluing station that are substantially similar to the previous ones.
A problem of dispensing multiple layers of media directly onto one another before forming a green product is that the media mix at the interfaces, so that the layers are not very well defined and have deviating properties, in particular pore sizes and porosities, in the region of the interface between two layers. This problem also occurs when layers of different character are sequentially compressed over a flat structure made previously, because the compression results in extra mixing of the media.